The present invention relates to a composition for prevention of electric waves in a wide range which is lightweight, flexible, tough and hardly flammable, is suitable as a material for prevention of electric waves, and is used for coping with radiation noises in electronic machines. More particularly, the present invention relates to a composition for prevention of electric waves which is useful as an electric wave absorber coping with all directions or as an EMI shield.
Electronic machines are used widely for various purposes and frequencies used therein range from several kiloheltzs (frequencies near those of direct current) to the GHz range (the frequency range of electric waves called microwaves). In addition, machines designed to radiate electric waves to the air, such as wireless machines, for example, portable telephones, wireless LAN systems, etc. have been markedly on the increase in recent years.
On the other hand, since electromagnetic waves radiated by these machines cause wrong operations of other electronic machines, there is a strong demand for the reduction of unnecessary radiation from them and the improvement of their resistance to extraneous electromagnetic waves. Moreover, there is pointed out the possibility that the radiated electromagnetic waves have undesirable influences on a human body. Thus, there is also a strong social demand for the reduction of unnecessary electromagnetic waves.
Typical examples of part used for such purposes are filters, shields, and electric wave absorbers. The filters protect machines by passing therethrough necessary signal components but repelling noise components without passage. The shields are such that the inside and outside of a machine to be protected is isolated from each other by surrounding the machine with an electroconductive film. The electric wave absorbers are characterized by converting incident electromagnetic waves to heat to prevent the generation of reflected waves.
Parts such as the filters, however, reflect noise components to return the same, so that the returned noise components are likely to have undesirable influences on other circuits and machines. Furthermore, at a frequency of the order of GHz, signal components are not only passed through a circuit but also radiated to the air in a high proportion, so that it becomes difficult to take an effective measure by using parts such as the filters.
As to the shield, it is almost impossible in practice to cover the machine completely, because of the presence of a signal-introducing portion, heat-radiating holes, etc. In addition, at a high frequency, radiated waves leak out through even a small gap. If the design of the shield is not suitable, the shield acts as an antenna, so that the amount of radiated waves is increased in some cases.
On the other hand, the electric wave absorbers can be ideal for coping with radiation noises because they convert incident electromagnetic waves to heat to prevent the generation of reflected waves. However, a frequency range in which each electric wave absorber is usable is limited depending on a material for the absorber. Therefore, the electric wave absorbers are not suitable for unnecessary radiation having spectrum components in a wide range. Electric wave absorbers usable in a wide frequency range have been developed as those for electric wave darkroom but cannot be used at all in electronic machines because of their large thickness of several tens centimeters or more.
Several examples of electric wave absorbers usable in various frequency ranges, respectively, have been disclosed. For example, Japanese Patent Kokai No. 58-73198 discloses an electric wave shutting-out box obtained by infiltrating or casting an electroconductive composite material matrix prepared by mixing a macromolecular material (e.g. plastic or rubber) with electroconductive carbon fiber, carbon black, graphite or metal powder and kneading them to effect dispersion, into a mat, cloth, net or flaky material of an electroconductive carbon-based fibrous material, a metallic fibrous material or a metallized non-metallic fibrous material. However, this invention increases the decrement of electric field strength in the range of 1 MHz to 100 MHz but is not effective against electric waves having a frequency of more than 100 MHz. Moreover, since the above-mentioned electric wave shutting-out box contains a metallic material and hence has a heavy total weight, its availability is limited.
Japanese Patent Kokai No. 60-249392 discloses an electromagnetic shielding material comprising a composition obtained by dispersing ferrite fine powder composed mainly of manganese and zinc, and electroconductive carbon fine powder in an organic macromolecular material. The composition has a ferrite fine powder content of 30 to 70 vol % and a volume resistivity of 10.sup.2 to 10 .mu..multidot.cm, and has electric wave absorbing properties. The composition, however can absorb only electric waves having a frequency in the range of 500 MHz to 1,000 MHz. Moreover, the composition is also limited in availability because it contains manganese, zinc, etc. and hence has a high specific gravity and a low flexibility.
In addition, Japanese Patent Kokai No. 5-21984 discloses a shielding composite material against electromagnetic waves obtained by dispersing in indefinite directions a plurality of coil-shaped carbon fiber pieces in cloth having a low electric conductivity (e.g. cement, synthetic resin, rubber or paper) to support them on the cloth. However, since the coil-shaped carbon fiber pieces tend to be intertwined with one another and hence are difficult to disperse uniformly in the cloth, the composite material is disadvantageous, for example, in that its shielding effect on electromagnetic waves is liable to be nonuniform. Therefore, the composite material is difficult to produce uniformly in a definite thickness or less.
The present applicant has previously applied for a patent on an invention relating to an electric wave absorber comprising graphitized carbon black composite particles composed of crystalline graphite and amorphous carbon black (see Japanese Patent Kokai No. 8-274493). However, this electric wave absorber is effective in absorbing electric waves having a frequency in the range of 50 MHz to 1,000 MHz but is not sufficient in ability to absorb electric waves having a frequency of less than 50 MHz or more than 1,000 MHz. In addition, the thickness of the absorber has to be set at a specific value for making the absorber usable at a specific frequency and hence may not be freely chosen. Furthermore, this absorber absorbs only electric waves coming from a definite direction and cannot absorb electric waves coming from all directions.
In a sheet-like electric wave absorber called a matching type electric wave absorber which has been on the market, or a conventional disclosed electric wave absorber, a conductor plate is set on the side reverse to the electromagnetic wave incidence side, and the energy of electromagnetic waves is converted to heat by utilizing also the effect of the interference of incident electromagnetic wave components and components reflected by the conductor plate, with each other. Therefore, the thickness of such an electric wave absorber is usually adjusted to one-fourth as large as the wavelength of electromagnetic wave. Such electric wave absorbers markedly absorb electromagnetic waves having a specific wavelength but hardly absorb electromagnetic waves having a different wavelength. Moreover, they absorb only electromagnetic waves coming from one direction and hence are not effective against noise components which have a wide spectrum and are various in the direction of their incidence. When used without a conductor plate, the matching type electric wave absorbers transmit electromagnetic waves with almost no absorption.
Thus, there have been reported absorbers having excellent absorption characteristics in a specific frequence range, but there has not yet been reported an absorber which has excellent absorption characteristics in a wide range and in all directions, and is flexible, thin and lightweight.